Motor driven scooters have had drives direct from a motor driven rotating shaft onto a driven wheel. Such drives give the driven wheel two points of wear. First, the driven wheel wears at its area of contact with the ground. Secondly, the driven wheel wears at the driving shaft. More importantly, the interaction between the driven wheel and the driving shaft is not positive. Slippage with accompanying friction losses make such transmissions less than optimal. For that reason, motor driven scooters and other small vehicles for the most part now rely upon sprocket and chain drives.
Unfortunately, chain drives for small all-terrain vehicles, such as scooters and go carts, are replete with problems. First, such vehicles operate in a dirt and mud environment. The resultant ambient grit produces high chain wear with resultant chain lengthening.
Second, chain lengthening due to chain wear can be easily understood. In the case of a chain having 94 links, chain wear for each link will occur at three separate places. First, each link is held together by a link pin. As the pin diameter decreases due to wear, the overall length of the link will increase in each chain direction by the amount of the wear. Second, each chain link includes forward-extending links and rearward-extending links. Each of these respective forward-extending and rearward-extending links fastens to the link pin at an aperture. Each of these apertures is subject to wear, especially in the grit environment. Each aperture as it is subject to wear becomes an individual contributor to chain lengthening. Because there are two apertures for each pin at each link, the chain wear at each aperture will contribute to chain lengthening. Thus, in the chain having 94 links, there are additively 94 pins and 188 apertures all subject to wear. Each wear point, being a pin or an aperture, lengthens the chain. Presuming that the small all-terrain vehicles are continually operated in a grit environment, adjustment for chain length change becomes an ongoing proposition.
A rapidly lengthening chain on a small all-terrain vehicle increases the probability of chain and sprocket derailment. Generally speaking, the smaller the chain, the more rapid the wear. The operator of the small all-terrain vehicle has an interest in maintaining proper chain tension.
It is known to use mechanical tensioning devices in such environments. However, such conventional mechanical tensioners require pivot points, spring bias, and chain idlers. They impose a considerable complication on a chain and sprocket drive. In the case of a small all-terrain vehicle, further complication of mechanical tensioning devices is disadvantageous, especially in the limited space available between the driving low-horsepower, high-speed motor and the sprocket-driven small-diameter ground-engaging wheel.
Small all-terrain vehicles typically use low-horsepower, high-speed motors. For example, in the scooter which forms a preferred example of this invention, a 2-½ horsepower 8000 rpm motor is used. This motor is used to drive wheels in the order of eight to nine inches. Rotation reduction is a key transmission system issue.
At the same time, small all-terrain vehicles place high dynamic loading on their transmissions. For example, where the wheels of such vehicles temporarily leave the ground and become airborne, return of the powered wheel to the ground normally produces high dynamic shock loads on the transmission system. As a result, many chain transmission systems have tried using chain sizes that can withstand the high dynamic shock loads. Unfortunately, with increased chain size, sprocket size and sprocket inertia increases. Increased sprocket size necessitates the use of a larger transmission system, requires the use of intermediate so-called idler or “jack” shafts, and increases transmission inertia, inhibiting acceleration and deceleration.
Intermediate idler or “jack” shafts present an especially undesired complication to chain and sprocket transmission systems for small all-terrain vehicles. In such idler or jack shafts systems, a first chain loops the high-speed drive sprocket at the low-horsepower motor to a second driven sprocket on the idler or jack shaft. A second chain loops the third drive sprocket on the idler or jack shaft and extends to a fourth driven sprocket at the small ground-engaging wheel. The additional mechanical parts of the idler or jack shaft and two sprockets, the additional second chain, the complexity of mounting the idler or jack shaft and the two sprockets, and the space required for such idler or jack shaft and two sprockets are generally unsuitable for small all-terrain vehicle chain transmissions.
Presuming that one wishes to use a small-size chain and sprocket drive for an all-terrain vehicle, the load limits of such small chains also become a problem. For example, a No. 25 chain has a tensile load limit in the order of 900 pounds (compared to the 2500-pound tensile load limit of the No. 35 chain). With normally available chain and sprocket transmissions, a lighter chain realizes greater probability of chain failure.
Finally, and presuming that one is going to use a small chain for such an all-terrain vehicle high-reduction chain and sprocket transmission, the transmission of power from a small high-speed sprocket to the small chain presents a power transmission issue. By definition, a small-diameter sprocket contacts the chain at a small number of lugs. In the typical chain and lug scooter drive, the total power of the engine is delivered to a small chain at a reduced number of lugs. The probability of lug failure and/or chain link failure increases directly proportional to the increased power transfer at each sprocket lug to each chain link.
We have utilized a chain keeper pivoting over a small driving sprocket as a chain tensioning apparatus. This chain keeper has a chain-contacting tongue elastically biased with respect to the keeper toward the inside of the chain loop. The chain-contacting tongue contacts and tensions the chain at the idle chain linkage between the small driving sprocket and the large driven sprocket. The chain keeper is a one-piece construction, preferably molded from a high-impact, wear-resistant, low-chain-slide-friction plastic material. This molded chain keeper has a chain-contacting tongue elastically biased to the slack side of the chain.
Vehicles driven by chain and sprocket drives typically have their motors rigidly mounted to the vehicle. The driven wheel and driven sprocket are typically adjustably mounted relative to the vehicle and thus adjustably mounted relative to the motor and driving sprocket. When chain elongation occurs, it is the wheel which is moved relative to the motor to provide the adjustment. Thus, the entire support of the wheel at the frame is changed as the chain elongates. And when the chain is replaced, movement of the wheel is repeated as the new chain wears. Typically, such mounts must be of the “fork type.” That is to say, the wheel is supported at both sides on its adjustable support. This double sided support of the wheel enables secure mounting of the wheel for supporting the weight of the scooter and rider when its position on the frame is changed.
Unfortunately, such adjustment of the wheel is incompatible with the modem tendency to provide single sided cantilever mounts for such driven wheels. Since the wheel is mounted on one side only, and not on both opposite sides, changing the position of the wheel relative to the motor is mechanically more complex.
Moreover, it will be understood that a driven wheel not only transmits the force required for the driving of the vehicle but additionally is loaded with the static and dynamic weight of the vehicle as it bears and impacts upon the ground. The adjustment of such a cantilevered wheel requires fastening of the wheel in its new position with sufficient support to resist both the dynamic forces occurring during driving of the scooter as well as the forces necessary to support of the vehicle. This firm type of mounting is at best difficult with a cantilever mount of the wheel.
Further, for such adjustment to occur, the vehicle must be supported free of the ground while the driving wheel is repositioned. This requires that the vehicle be held off the ground while repositioning of the wheel occurs. Simple, in the field adjustment, cannot occur. The scooter must be taken to a repair platform where the adjustment can occur.